Over the past five years, I have been involved in detailed neuroanatomical studies of the amphibian central nervous system. The particular emphasis of these studies has been on those circuits which play a role in visuo-oculomotor integration. The long term objectives of these studies is to develop a major overview of the circuitry underlying vertebrate visuo-ocular reflexes and to provide a new perspective on the range and degree of interclass differences. In the present proposal a series of experiments are described which will expand our understanding of reptilian visuo-oculomotor integration. Specifically the histochemical tracer, horseradish peroxidase (HRP) will be used to determine the organization of the pretectal nucleus lentiformis mesencephali in both turtle and lizard. This portion of the pretectum has been implicated in the control of optokinetic nystagmus in both amphibians and mammals, but little is known about its anatomical and functional organization in reptiles. In the second phase of the project the HRP technique will be used to determine the organization of the cell groups surrounding the oculomotor nuclei in turtle and lizard. The visual field-holding response integrates the inputs to the oculomotor nuclei from three modalities: vision, position sense and vestibular sense. Only through an understanding of the neural circuits of these systems can we better understand the functional organization of this complex sensory-motor transition. Nonmammals are well suited for investigations of sensory-motor integration due to the relatively simple nature of their nervous systems. Impairment of these systems in Man leads to a wide variety of neurological disorders, from congenital nystagmus and oculomotor apraxia, to a variety of oculomotor and vestibular abnormalities for which adequate neural models have yet to be described.